When digital video such as MPEG (Moving Pictures Expert Group) digital video is played back in various trick play modes, access to I frames, P frames or B frames is generally required. I frames, P frames or B frames contain information that can be used to directly represent an image, whereas images are created with B frames by referencing and modifying preceding and/or subsequent I frames and P frames. As an example, when playback is carried out at 2× or 4× speed, the playback algorithm jumps from I frame to I frame or P frame to P frame to speed up the presentation of the video frames while skipping certain frames. B frames are needed for slower than 1× speeds.
Unfortunately, when content is encrypted using traditional encryption techniques, trick play becomes difficult or impossible since content has to be decrypted in order to find the I frames, P frames and/or B frames. This therefore makes it impossible to record the content with the same encryption as delivered over the delivery system. In the case of unencrypted content, these frames are accessed by calculating an offset from the start of frame (SOF). In U.S. Pat. No. 7,151,833 to Candelore, et al., selective encryption is used as a remedy to this problem. In accord with certain embodiments disclosed in this patent a method of selectively encrypting and decoding digital video content is provided in which certain of the packets are selected for encryption according to a selection criterion, wherein the selected packets exclude packets containing start of frame (SOF) headers. The selected packets are encrypted while retaining the packets containing the SOF headers unencrypted to form selectively encrypted digital video content. In recording selectively encrypted digital video content to a storage medium, offsets are calculated for at least one of I frames and P frames from the SOF headers and the calculated offsets are stored as an offset table. The offset table can then be accessed to enable trick play of the selectively encrypted digital video content. In this manner, trick play can be accomplished expediently.